Researchers have made a startling discovery: Americans are shrinking. A nation once famed for its strapping, well-nourished youth is gradually diminishing in physical stature.

By contrast, the heights of men and women from Europe are increasing inexorably. The average Dutchman, whose country produces the Continent's loftiest men, is now more than six feet tall - almost two inches above his American counterpart. And he is still growing. Across the Netherlands hotel owners are lengthening beds and raising door mantles to stop the nation's tall youth suffering from irreparable anatomical damage.

According to a New Yorker essay on the subject last week, Dutch ambulances are even having to keep their back doors open on many occasions to allow for the prodigious dimensions of their patients' legs.

New research has shown some unexpected disparities between statures of Americans and Europeans, indicating that recent social changes and diet are major influences on adult height.

For British men, too, are outstripping their transatlantic rivals. At the time of the American Revolution, the average US male was two inches taller than his British counterpart. Today he is almost half an inch shorter.

This surprising reappraisal of American and European physiques is the work of researcher John Komlos of Munich University. 'Much of the difference is due to the great social inequality that now exists in the United States,' Komlos told The Observer last week. 'In Europe, there is - in most countries - good health service provision for most members of society and plenty of protein in most people's diets. As a result, children do not suffer illnesses that would blight their growth or suffer problems of malnutrition. For that reason, we have continued to grow and grow.'

On the other hand, America has eight million people with no job, 40 million individuals with no health insurance, 35 million living below the poverty line, and a population that exists mainly on junk food. There, the rise in average height that marked its progress as a nation through the 19th and 20th centuries has stopped and has actually reversed - albeit very slightly - in recent years. Many Americans are rich and do well anatomically as a result, but there is a large underclass that is starting to drag the country down the stature charts.

This discovery, which has been revealed through research that Komlos has assembled over decades, amounts to an assault on the values of the free market economy espoused by Americans and provides powerful support for those who back European ideas about universal healthcare.

Fluctuations in human stature are not new and have occurred regularly throughout history. Our early hunter-gatherer ancestors were tall and lean. Later, as farming spread across the world, dense populations learnt to live on only a few standard crops and suffered considerable nutritional neglect. The result was a decline in stature.

Similarly, climatic changes have had a profound effect on human height - a physical attribute that is now regarded by historians, scientists and economists as a key indicator of the health of any group of people living at any particular time and place. For example, during the Little Ice Age, in which temperatures plummeted across the world between 1300 and the mid-19th century, there was a noticeable decrease in human stature.

'There are two possible mechanisms for this observation,' said Professor Chris Stringer of the Natural History Museum. 'Firstly, all mammals get shorter and rounder when climates cool. It is a physiological response to cold. Short, round bodies preserve heat better than tall cool ones.

'However, there is an alternative explanation for shrinking stature in bad weather. It means poor crops, and that in turn means malnutrition and, of course, the consequence of that is poor stature.

'Those tiny suits of armour that you see when you visit the Tower of London were worn by people who were badly nourished. During winter they would have only salt meat and a few vegetables to live on. That's not going to help you grow very well.'

Modern society now protects humans from such problems, though in recent years it has become clear that political factors are having some effect on nutrition levels and proper diet and therefore stature. And it is in this arena that Komlos has made his key discoveries.

Through painstaking investigations he has calculated the heights of men at different times over our recent historical past. This shows that, around 1850, Americans - blessed with Western technology that allowed its citizens to spread unstoppably across the United States - lived relatively fine lives that let its menfolk reach an average height of 5ft 9in. By contrast, Dutchmen were only able to reach about 5ft 7in.

By the early 20th century the average American man was still about the same height as his predecessor. But the average Dutchman had nearly caught up and was only about half an inch shorter.

But in the 20th century Americans were overtaken. The average US male is now about 5ft 10in. The average Dutchman is just over 6ft.

More importantly, the latter is continuing his rise in average height. The Americans have long since stopped growing and, according to some measures, may actually be getting smaller. 'In relative terms, Americans are certainly shrinking in comparison with Europeans,' says Komlos.

One possible explanation lies with immigration. As more Mexicans and Chinese enter the US, these individuals may lower the average height, it is argued. But statisticians dismiss this suggestion. During the 19th century the country took in millions of malnourished, and therefore small, people. Yet Americans remained the tallest people in the world at that time.

In fact, the very idea that various peoples are programmed, on average, to be short or tall is thrown into doubt by Komlos's work. Apart from a few rare races, such as African pygmies who are genetically programmed to have low stature, virtually everyone in the world has the potential to reach the same average height as the Dutch, and that includes the Mexicans, Chinese, Inuit, and other peoples who are not usually noted for their stature.

To achieve that status will require some arduous social engineering. The Dutch health service, with its magnificent support services for pregnant woman (quality of life in the womb is a key factor in determining future health and height) and its high-protein diets based on dairy food, will not be easy to emulate in a world whose population is now soaring towards seven billion.

Growth Spurt

The steady increase in the height of British youngsters which began over a century ago continues to manifest itself today, particularly among girls.

According to figures provided by the Child Growth Foundation, the height of the average British nine-year-old girl rose from 4ft 4in (130.6cm) in 1983 to 4ft 5in (132.7cm) in 2003, while the average British nine-year-old boy increased from 4ft 4in to just under 4ft 5in.

'It would appear that girls are doing slightly better than boys, but that may simply be due to the fact that girls are reaching puberty earlier and earlier, and are beginning their adolescent growth spurt before boys,' said Tam Fry, of the foundation.

The increase is also roughly in line with the general trend observed over the past 100 years that the average child is increasing in height at a rate of between four and five inches a century.